In a quantum communication system, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarization, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution (QKD) is a technique which results in the sharing of cryptographic keys between two parties: a transmitter often referred to as “Alice”; and a receiver often referred to as “Bob”. The attraction of this technique is that it allows to quantify the maximum information potentially known to an unauthorised eavesdropper, often referred to as “Eve”. In many forms of QKD, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine the potential information gained by Eve.
Measurement device independent QKD, MDI-QKD has also been developed for the situation where the security of the measurement devices (Bob) might be in doubt. In MDI-QKD, two senders, usually Alice and Carl, send quanta to Bob. Alice and Carl can distil a secret key from the publicly announced measurements that have been made by Bob.